The Medical Examiner's Office has not yet revealed the identity of the man with the knife. He is believed to be in his 50s.

One theory, advanced by police, is that it was a situation involving an aggressive panhandler that escalated. But that's not confirmed. The first calls to police were about a man threatening people with a knife.

The incident took place on State and Randolph at lunch time. People were everywhere. Police were called because a man was waving a knife around. They tried to subdue him but that failed, as did pepper spray. The man grabbed a woman and her purse. Then he took hold of a man and put a knife to his head. Police intervened.

"They were telling him, 'sir, drop the knife. Just drop the knife, put your hands up and drop it,'" said Angelo Watts, witness.

The suspect refused. Police fired at least four times. The man was then killed. His knife falls to the pavement.

Passersby, some of whom dove for cover, were in shock.

Onna Ford-Kendrick was driving through the intersection and saw the whole thing. She was about to pick up her daughter from a dance rehearsal at the Joffrey Ballet.

"I didn't know if she was involved. I was terrified. Paralyzed really," said Ford-Kendrick.

Among the officers who responded are partners assigned to Loop traffic. One of the shots fired by one officer hit his partner. The slug was stopped by the protective vest he was wearing.

"The officer who was hit in the vest actually had hands on the individual trying to disarm him. The other officer in ear of both his partner's life and the innocent victim, used his weapon striking both the offender, and his partner officer," said Supt. Jody Weis, Chicago Police.

The officer who caught the bullet in his vest was treated and released. And the man who had the knife to his head was not physically hurt.

"It's crazy. You would never think this would happen downtown," said Matthew Banks, witness.

Just after the shots rang out, Onna Ford Kendrick curbed her car, and sought safety. Her daughter is fine, but as she passed the intersection earlier on Thursday she didn't know that, and she also immediately thought of her husband - a Chicago cop - twice wounded in his career. It all replayed in her head on Thursday night, as it probably is with other witnesses.

Since it was a police involved shooting, the Independent Police Review Authority is investigating.